This invention describes an improved method for the efficient recovery of metal ions such as copper ions from aqueous solution by use of a supported liquid membrane.
Copper may be extracted from a variety of liquid compositions such as copper-containing waste solutions, spent cupric and alkaline streams resulting from copper etching, rinse water from copper electroplating processes, copper-containing solutions resulting from the precipitation of basic coppers salts, as well as production of a micronutrient supplement as described in U.S. Pat. No. 5,451,414.
The concentration of copper in such compositions generally ranges from about 5-50xc3x9710xe2x88x922 g/l, and more generally from about 1-2xc3x9710xe2x88x921 g/l. U.S. Pat. Nos. 3,927,169; 3,988,151; 4,083,758; 4,336,231; 5,670,035; and 5,895,633 disclose the recovery of copper from relatively concentrated solutions ( greater than 2 g/l Cu) by liquid/liquid extraction. However, the concentration of copper which is usually present in copper-containing waste solutions is too low for profitable application of liquid/liquid extraction and too high for application of ion exchange. Indeed, Tavlarides et al. in xe2x80x9cSolvent Extraction, Membranes and Ion Exchange in Hydrometallurgical Dilute Metals Separationxe2x80x9d state that solvent extraction is uneconomical in the handling of large volumes of solution containing low metal values.
Liquid membrane separation, which combines the steps of solvent extraction with membrane separation followed by stripping from the membrane in a single process, is generating special interest because of its potential for low cost and energy saving. One type of liquid membrane system is supported liquid membrane (SLM). SLM employs a solvent/carrier mixture contained within the pores of a polymeric membrane support. In an SLM process, a metal ion may be selectively removed from an aqueous metal ion-containing feed solution by reaction of the metal ion with the carrier to form a metal ion-containing complex in the membrane impregnated with the solvent/carrier mixture. The membrane is interposed between the metal ion-containing feed solution and an aqueous stripping solution. The metal ion-containing complex diffuses through the membrane and metal ions are then stripped from the complex at the other side of the membrane by contact with the stripping solution. Sulfuric acid is one example of a stripping solution that may be employed in an SLM process. The pH gradient between the feed and stripping solutions is the driving force for the SLM process.
The removal of metal ions by use of supported liquid membranes is described in U.S. Pat. Nos. 5,114,579, 4,661,257 and 4,617,125, each herein incorporated by reference.
More specifically, in a typical SLM process Men+ ions are transferred from the feed to the stripping solution and exchanged to nH+ ions of the stripping solution whereby the pH of the solution decreases over time. As a result, the driving force of the process changes with time. The resulting decrease of pH of the feed solution reduces the effectiveness of the extraction of metal ions from the feed solution and reduces the rate of the SLM transfer. A typical SLM process needs xe2x80x9con linexe2x80x9d pH adjustment in order to maintain the driving force of SLM process. From this point of view the SLM process is similar to liquid/liquid extraction. Garry Kordosky et al. xe2x80x9cThe Use of pH Control in Solvent Extraction Circuitxe2x80x9d, Henkel Corporation, 1979, describe the technique and advantages of pH controlled liquid/liquid extraction. Generally, laboratory experiments are conducted using a flat sheet membrane module and the cell can be equipped by simple pH-stat for control. It is much more difficult to achieve a pH controlled pilot scale SLM process.
Large-scale SLM processes are conducted in an apparatus having a high surface/volume ratio, for example, by use of hollow fiber modules. The pH of the feed solution flowing through the hollow fibers changes dramatically over time and it is not possible to adjust the composition of the feed solution during transport through the hollow fibers. The inability to maintain maximal driving force during the SLM process leads to a decrease of the removal rate and may lead to a result where it is not possible to achieve complete removal of targeted ions. A solution to such a problem is proposed in U.S. Pat. No. 5,868,935, where a decrease in pH associated with the extraction of a cation into a solvent with a carrier, having an exchangeable proton, is offset by simultaneous removal of H+ ions together with a different ion, having an opposite charge, through another membrane into a different solution. Unfortunately, it is not common for a different suitable anion and cation species to be simultaneously present in metal-containing aqueous waste solutions. The process itself becomes more complicated because of the necessity of using two different membranes.
U.S. Pat. No. 5,188,703 discloses a method for the recovery of copper recovery from spent ammoniacal etchant. A complex salt of copper is precipitated by the addition of hydrochloric acid. However, around 1,000 ppm of copper together with a high concentration of NH3 remains in solution.
A description of the simultaneous processing of a copper containing ammoniacal and acidic solution can be found in U.S. Pat. No. 5,451,414, which discloses a process of micronutrient supplement manufacturing by mixing spent ammoniacal and acidic etchants together with the precipitation of copper basic chloride, which is used as a micronutrient supplement. About 500 ppm (0.5g/l) copper accompanied by a high concentration of NH3 remains in solution in this process.
It is accordingly one object of the present invention to provide an improved process for the recovery of metal ions from an aqueous feed solution by use of a supported liquid membrane.
It is further an object of the present invention to provide a process for the removal of metal ions from an aqueous feed solution by use of a supported liquid membrane which allows effective removal of the metal ions without adjustment of the composition of the feed solution during removal of the metal ions.
It is another object of the present invention is provide an efficient process for the removal/recovery of metal ions from dilute aqueous solutions whereby the metal concentration is too low for solvent extraction and too high for ion exchange to be practical.
It is still yet another object of the present invention to provide an efficient and highly effective process for the removal of copper ions from aqueous feed solution by use of a supported liquid membrane.
In accordance with the present invention, there is thus provided an improved process for the recovery of metal ions from metal-containing aqueous solutions using a supported liquid membrane (SLM) whereby the pH of the metal-containing solution is adjusted to between 5.5 and 8 and ammonia or other weak base or salt thereof is present in the metal-containing solution, preferably in a concentration of from 0.5 to 8 M, whereby complexes are formed between the ammonia or other weak base or salt thereof and metal ions present in the solution. The present invention permits enhanced recovery of metal ions from solution without any need to adjust the pH of the solution during the SLM process.